1. Field of the Invention
The present invention relates to an optical scanner with a heat transfer function and an image forming apparatus including the same, such as a digital copier or a laser printer.
2. Description of the Related Art
Image forming apparatuses are increasingly capable of producing images with higher speed and higher density. Accordingly, image forming apparatuses are equipped with optical scanners including polygon scanners that rotate at a rotational speed of 30,000 rpm-50,000 rpm.
When polygon scanners rotate at high speed, heat is generated due to friction of a rotation driving unit. Furthermore, the current increases in the circuit, which causes even more heat. As a result, the temperature near the polygon scanner rises. Moreover, a hissing sound caused by the rotation of a polygon mirror becomes louder.
Optical boxes are often made by molding a low-cost resin material. However, because thermal conductivity of resin is low, it is difficult for heat to be released outside from such an optical box made of resin, when the temperature rises near the polygon scanner. This shortens the service life of the polygon scanner and/or deforms the optical box and/or an imaging element, thus degrading optical properties.
One approach is disclosed in Patent Document 1. Specifically, a deflector (rotating mirror) is covered by an aluminum die-cast cap in such a manner that part of the cap is exposed to the atmosphere outside an optical scanner. However, such a metal cap is extremely expensive. Furthermore, the deflector is usually arranged in the center of the optical scanner in a main scanning direction. Therefore, it is difficult to lay out a flow passage such that airflow for cooling the exposed part of the cap is guided toward the cap and the warmed air colliding against the cap is released outside.
In one example of an optical scanner, plural light fluxes from different directions are incident on one deflector and are deflected by the deflector. The light fluxes then pass through plural imaging elements arranged substantially symmetrically with respect to the deflector to scan plural surfaces (imaging surfaces). In such an optical scanner, the deflector is arranged in the center of a large area of the optical scanner. Therefore, it is extremely difficult to secure a sufficient amount of space for installing a duct and a fan for guiding airflow toward the deflector. Accordingly, even if extra funds are spent and a large space is reserved for providing radiating fins near the deflector, a sufficient cooling effect cannot be achieved. Furthermore, in an effort to reduce the size of the image forming apparatus, components are increasingly being arranged close to each other, which is disadvantageous in terms of the layout for heat radiation. Moreover, the usage of a cooling fan increases power consumption and generates noise, which have a great impact on the environment.
Meanwhile, in a method disclosed in Patent Document 2, the space around the deflector is not enclosed, so that the air in the entire space inside an optical scanner can be mixed with airflows generated by the rotation of a deflector. Therefore, the temperature of the deflector does not rise. However, in this method, the warmed air near the deflector is directly blown onto a scanning imaging element, and the temperature inside the entire optical scanner changes rapidly. As a result, the scanning imaging element may be deformed due to changes in the temperature, and optical properties are considerably degraded due to changes in the positions and/or tilt angles of the optical elements. Furthermore, the hissing sound caused by the rotation of the deflector is not sufficiently reduced, and the noise can be heard outside the optical scanner.
Patent Document 1: Japanese Laid-Open Patent Application No. H9-105881
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-92119